SrnR from Streptomyces griseus is a nickel-binding transcriptional activator

Nickel ions are crucial components for the catalysis of biological reactions in prokaryotic organisms. As an uncontrolled nickel trafficking is toxic for living organisms, nickel-dependent bacteria have developed tightly regulated strategies to maintain the correct intracellular metal ion quota. These mechanisms require transcriptional regulator proteins that respond to nickel concentration, activating or repressing the expression of specific proteins related to Ni(II) metabolism. In Streptomyces griseus , a Gram-positive bacterium used for antibiotic production, Sg SrnR and Sg SrnQ regulate the nickel-dependent antagonistic expression of two superoxide dismutase (SOD) enzymes, a Ni-SOD and a FeZn-SOD. According to a previously proposed model, Sg SrnR and Sg SrnQ form a protein complex in which Sg SrnR works as repressor, binding directly to the promoter of the gene coding for FeZn-SOD, while Sg SrnQ is the Ni(II)-dependent co-repressor. The present work focuses on the determination of the biophysical and functional properties of Sg SrnR. The protein was heterologously expressed and purified from Escherichia coli . The structural and metal-binding analysis, carried out by circular dichroism, light scattering, fluorescence and isothermal titration calorimetry, showed that the protein is a well-structured homodimer, able to bind nickel with moderate affinity. DNase I footprinting and β-galactosidase gene reporter assays revealed that apo- Sg SrnR is able to bind its DNA operator and activates a transcriptional response. The structural and functional properties of this protein are discussed relatively to its role as a Ni(II)-dependent sensor.